1. Field of the Invention
The present invention relates to a semiconductor device, a method for making the same and an apparatus for making the same.
2. Related Background Art
One of the essential steps in the preparation of a semiconductor device is oxide film formation. The oxide film is generally formed by thermal oxidation. FIG. 1 illustrates a conventional apparatus for making the semiconductor device. Silicon wafers 701, from whose surface trace amounts of heavy metals and alkali metals present thereon are removed by acid treatment and deionized water with impurity content reduced to the lowest minimum, are placed on a boat 702 of highly pure quartz and are inserted into the central part of an oxidation oven, by means of a pulling rod 703 of highly pure quartz. Into a tube 704 of highly pure quartz, constituting the oxidation oven, there is introduced process gas 705, consisting of highly pure oxygen, a gaseous mixture obtained by combustion of hydrogen and oxygen, or a mixture of such oxidative gas and HCl. Said process gas 705 may be diluted with inert gas such as nitrogen, in order to control the oxidation rate. The tube 704 is heated by a heater 706 from the outside, and the central part is precisely temperature controlled. Between the tube 704 and the heater 706, there is inserted a line tube 707 of highly pure SiC, serving to prevent the contamination from the heater 706 to the tube 704 and to attain uniform and stable oven temperature. The silicon wafers 701, heated to about 800.degree. to 1200.degree. C., are thus oxidized by the process gas 705, whereby a thermally oxidized film is formed on the surface. The thickness of the thermal oxidation film is determined by the temperature of the oxidation oven, the composition, mixing ratio and dilution ratio of the process gas and the process time.
A scavenger 708 is provided to recover remaining gas 709 that has not contributed to oxidation, and to protect a clean bench area 710 from a large amount of heat liberated from the oxidation oven. A duct 711 for recovering the exhaust gas 709 is connected to the scavenger 708.
However, the film forming operation in the above-explained apparatus has been associated with a drawback of formation of a spontaneous oxide film, thereby deteriorating the characteristics of the prepared semiconductor device. The present inventors have made investigations for the cause of the above-mentioned drawback, and have obtained following knowledge:
(1) Conventionally, in order to suppress the heat liberated to the clean bench area 710, the gas suction by the duct 711 has been conducted strongly, in excess (for example in the order to several tens times larger) of the amount required for removing the exhaust gas 709. For this reason, in the popular open-tube oxidation oven, there is involved external air which generates spontaneous oxide film; PA1 (2) Even if the gas exhaust by the duct 711 is suppressed, the oxidation employing gaseous mixture obtained by combustion of oxygen and hydrogen in the popular open-tube oxidation oven leads to the formation of a laminar flow, deteriorating the thickness distribution of the oxide film. PA1 (1) The crystalline character of the metal electrodes is affected by the surfacial orientation of the semiconductor substrate; and PA1 (2) The current-voltage characteristics of the semiconductor-metal contact are determined by the surfacial orientation of the semiconductor substrate and the electrode material. PA1 1) reduction of the step difference; PA1 2) reduction in the reflectance of the thin films; and PA1 3) use of photolithographic technology which is not affected by the halation. Although these measures are proper, they require complication and more precision in the process, thus inevitably resulting in an increased cost. PA1 I.sub.Bdiff : diffusion current of positive holes injected from the base into the emitter; PA1 I.sub.Brec : recombination current of positive holes recombined inside the base; and PA1 I.sub.rec : recombination current in the depletion area. PA1 (1) a semiconductor device provided with a thin film capable of passing the tunneling current and a polycrystalline layer laminated on said thin film and constituting an energy gap with the emitter area for suppressing or inhibiting the injection of the minority carriers, wherein the thickness of the emitter area is selected smaller than the diffusion length of the minority carriers; and PA1 (2) a semiconductor device employing microcrystals or an amorphous semiconductor as the emitter area and an energy band gap of the emitter area wider than that of the base area.
FIG. 2 shows the oxygen concentration at a position of 1 mm from the open end of a quartz tube of an internal diameter of 180 mm, when highly pure nitrogen almost free from moisture or oxygen is introduced, as a function of open or closed state of the scavenger duct. Other conditions are shown in FIG. 2.
According to FIG. 2, the oxygen concentration is 5.4% at a nitrogen flow rate of 10 l/min, when the scavenger duct is opened. Such oxygen concentration is extremely high, in consideration of facts that the oxygen concentration in the air is about 20% and that highly pure nitrogen is introduced into the quartz tube. These facts indicate a large involvement of the external air by the scavenger duct. FIG. 2 also indicates that such external air involvement can be prevented to a certain extent if the scavenger duct is closed, but such state is not practical because the temperature of the clean bench area is elevated.
On the other hand, a closed-tube configuration can prevent the external air involvement at the oxidizing operation. However, even in such closed-tube configuration, the oven becomes temporarily open at the insertion and extraction of the wafers, and the spontaneous oxide film is formed in the period until the wafers are inserted into the central part of the oven. Recent miniaturized semiconductor devices require a thickness of the gate oxide film not exceeding 100 .ANG., and the presence of such spontaneous oxide film affects the characteristics of the semiconductor devices.
On the other hand, in the conventional semiconductor devices, the orientation of a semiconductor at a junction interface of the metal-semiconductor contact is determined by the surface orientation of the semiconductor substrate, and the electrode are generally composed of the same metal as that employed for the wiring material.
However, lack of selection of the optimum surfacial orientation of the semiconductor and of the optimum metal material has resulted in the following drawbacks in the manufacturing method and the function of the semiconductor devices:
Also, in the conventional semiconductor integrated circuits, there are provided wiring layers extending along the surface of the cells for electric connection of said cells, but such wiring layer is apt to cause disconnection at the step difference at the boundary between the cell and the exterior. The causes for such disconnection are the step difference in the height, at the boundary, between each cell and the surrounding area, and the halation induced by various highly reflective thin films present in the vicinity of said boundary of the cell.
Such disconnection has conventionally been prevented by improvements in the process, such as:
Also, since all the bipolar transistors formed on a same substrate have a substantially same emitter-grounded yield voltage, the yield voltage of all the bipolar transistors in the designing of a semiconductor device is determined by that of a bipolar transistor requiring the highest yield voltage.
However, in case of forming bipolar transistors belonging to two or more power sources on a same chip, the distance between the buried layer and the base, determining the above-mentioned yield voltage and the cut-off frequency of the bipolar transistors, has to be made large so as to match a higher power source voltage, so that the cut-off frequency of the bipolar transistors belonging to the power source of a lower voltage becomes inevitably low.
Also in case bipolar transistors are employed as analog amplifiers or in a photo sensor of bipolar configuration, there are required those showing little change in the current gain H.sub.FE over a wide collector current range.
However, the conventional bipolar transistors tend to show a larger variation in the current gain, resulting from a variation in the collector current, with the size reduction, so that it has been difficult to obtain the bipolar transistors showing little change in the current gain H.sub.FE over a wide range of collector current.
The above-mentioned drawback becomes particularly conspicuous in the bipolar transistor in which the injection of the minority carriers from the base area into the emitter area is suppressed in order to achieve a high-speed transistor function, a higher current amplification gain and a higher frequency limit.
This point will be explained further in the following, with reference to FIGS. 3 and 4. The base current I.sub.B of the bipolar transistor can be represented by the following equation: EQU I.sub.B =I.sub.Bdiff +I.sub.Brec +I.sub.rec
wherein:
Among these components, I.sub.Bdiff and I.sub.Brec are related to the emitter-base voltage V.sub.BE in the following manner: EQU I.sub.Bdiff, I.sub.Brec .varies. exp (V.sub.BE /KT)
while I.sub.rec is related in the following manner: EQU I.sub.rec .varies. exp (V.sub.BE /2KT)
wherein K is the Boltzmann constant and T is the temperature.
Also the collector current I.sub.C is related in the following manner: EQU I.sub.C .varies. exp (V.sub.BE /KT)
Since I.sub.Bdiff &gt;&gt;I.sub.Brec, I.sub.rec in the conventional bipolar transistors, the collector current I.sub.C or the base current I.sub.B, when represented by the ordinary logarithm as a function of the emitter-base voltage V.sub.BE, assumes the form of a straight line of an inclination (log e)/KT as shown by solid lines in FIG. 3. In a high current region, however, the collector current and the base current are no longer proportional to the emitter-base voltage by the factor exp(V.sub.BE /KT) because of conductance modulation and other effects.
The current amplification gain H.sub.FE of the bipolar transistor, being defined by I.sub.C /I.sub.B, is ideally constant regardless of the collector current.
In practice, however, with the size reduction of the bipolar transistor, the recombination current I.sub.rec in the depletion area becomes no longer negligible with respect to the diffusion current I.sub.Bdiff of the positive holes injected from the base into the emitter, and the base current, being affected by a component proportional to exp(V.sub.BE /2KT) as represented by a broken line in FIG. 3, assumes the form of a curved line as indicated by a chain line therein. The current amplification gain, being defined by I.sub.C /I.sub.B as mentioned above, becomes lower in such case in a region where the collector current is low.
In order to reduce such current gain dependence of the collector current I.sub.C, I.sub.rec has to be suppressed at a low level.
Also the aforementioned bipolar transistor in which the injection of the minority carriers from the base area into the emitter area is suppressed is apt to be affected by the recombination current I.sub.rec in the deplection area because the diffusion current I.sub.Bdiff of the positive holes injected from the base into the emitter is small, so that the control of the recombination current I.sub.rec in the depletion area is essential for reducing the current gain dependence of the collector current I.sub.C.
Examples of such bipolar transistor in which the injection of the minority carriers from the base area into the emitter area is suppressed include:
In the following there will be explained, with reference to FIG. 4, why the recombination current I.sub.rec in the depletion area becomes not negligible with the size reduction of the bipolar transistor. FIG. 4 is a cross-sectional view of a bipolar transistor, in the vicinity of the emitter area thereof.
In FIG. 4 there are shown a collector area 101 constituting a second main electrode area of a second conductive type; a base area 102 constituting a control electrode area of a first conductive type; an emitter area 103 constituting a first main electrode area of the second conductive type; a lead electrode 104, composed of Al or polysilicon, for the emitter area; and an insulation film 105 composed for example of silicon oxide. In the transistor of such configuration, a depletion area 106 of a metallurgic junction in nature is formed in the vicinity of the junction between the base area 102 and the emitter area 103, and a surfacial depletion area 107 is formed in the vicinity of the interface between the base area 102 and the insulation film 105.
Said surfacial depletion area 107 is formed for example by the fixed charge and the movable ionic charge in the insulation film 105, the trapped charge at the interface between the insulation film 105 and the base area 102, and the difference in work function between the electrode 104 formed on the insulation film 105 and the base area 102.
The above-mentioned recombination current I.sub.rec in the depletion area is generated in the depletion area 106 of metallurgic junction and the surfacial depletion area 107.
In the aforementioned equation, the diffusion current I.sub.Bdiff of the positive holes injected from the base into the emitter and the recombination current I.sub.rec of the positive holes recombined in the base area are respectively proportional to the size of the emitter area.
On the other hand, within the recombination current in the depletion area, that generated in the depletion area of metallurgic junction is proportional to the size of the emitter area, but that generated in the surfacial depletion area is not proportional to said size.
Consequently, along with the size reduction of the transistor or the emitter area thereof, the recombination current generated in the surfacial depletion area occupies a larger proportion in the base current.
For this reason, in order to suppress the recombination current I.sub.rec in the depletion area, it is necessary to control the recombination current generated in the surfacial depletion area.
Said recombination current generated in the surfacial depletion area can be almost completely eliminated by surface state control so as not to form the depletion area at the surface of the semiconductor substrate.
Stated differently, in order to reduce the collector current dependence of the current amplification gain of the bipolar transistor, the surface state of the semiconductor substrate has to be controlled.